1. Field of Invention
This invention relates to an electrophotographic printing machine with a high-performance image transferring unit.
2. Description of Related Art
An electrophotographic printing machine has conventionally employed an image transferring system which utilizes corona discharge to transfer a toner image from a photosensitive material onto a recording medium. However, the system has the disadvantage of losing image transferring capability when there is high humidity in the environment. The image transferring capability is also affected when the corona wire is contaminated or the recording medium is creased. In order to eliminate these difficulties, an electrophotographic printing machine has been proposed in which a recording medium onto which an image is to be transferred is pushed against the photo-sensitive material by an endless belt with a volume resistivity of 10.sup.9 to 10.sup.14 .OMEGA..cm, and the charge on the endless belt is maintained with a corona charging unit on the inner surface of the endless belt. In the conventional electrophotographic printing machine, the image transferring characteristics are greatly affected by the variation in electrical characteristics of the endless belt.
The conventional electrophotographic printing machine will be described with reference to FIG. 4 in more detail. As shown in FIG. 4, an endless belt 1 is laid over a driving roller 2a and driven rollers 2b and 2c, and pulled tight by a tension roller 7 so that a recording medium 6 to which an image is to be transferred is pushed against a photo-sensitive material 3. In order to satisfactorily transfer the image onto the recording medium, the surface potential of the endless belt should be maintained at a suitable value. However, ionic materials formed by the corona discharge of a charging unit 18 or dust in the air are liable to stick to the inner surface of the endless belt 1. This can cause the electrical resistance and dielectric constant of the endless belt to change with time, so that the surface potential is decreased and the amount of flow of charges into the recording medium changes, thus adversely affecting the capability of the endless belt in both its image transferring capabilities and its sheet conveying capabilities. All of these disadvantages of the conventional system result in an endless belt with a relatively short service life.